1. Field of the Invention
This invention relates to nonvolatile memory elements, and more particularly, to methods for forming resistive switching memory elements used in nonvolatile memory devices.
2. Description of the Related Art
Nonvolatile memory elements are used in systems in which persistent storage is required. For example, digital cameras use nonvolatile memory cards to store images and digital music players use nonvolatile memory to store audio data. Nonvolatile memory is also used to persistently store data in computer environments.
Nonvolatile memory is often formed using electrically-erasable programmable read only memory (EEPROM) technology. This type of nonvolatile memory contains floating gate transistors that can be selectively programmed or erased by application of suitable voltages to their terminals.
As fabrication techniques improve, it is becoming possible to fabricate nonvolatile memory elements with increasingly smaller dimensions. However, as device dimensions shrink, scaling issues are posing challenges for traditional nonvolatile memory technology. This has led to the investigation of alternative nonvolatile memory technologies, including resistive switching nonvolatile memory.
Resistive switching nonvolatile memory is formed using memory elements that have two or more stable states with different resistances. Bistable memory has two stable states. A bistable memory element can be placed in a high resistance state or a low resistance state by application of suitable voltages or currents. Voltage pulses are typically used to switch the memory element from one resistance state to the other. Nondestructive read operations can be performed to ascertain the value of a data bit that is stored in a memory cell.
Current ReRAM devices are generally created in a MIM structure (metal-insulator-metal) using nonvolatile memory as the insulator. It is desirable to construct these devices in a crossbar array structure with each word line and bit line addressed by a driver transistor. As such, the bit at each intersection requires its own leakage-reduction element, such as a diode that serves as a current steering device. The diode only controls passage of current in one direction or the other, but it does not suppress the current. To prevent over-programming of the memory device, an additional resistor may be required at each node.
Manufacturing constraints, however, place limitations on the type of resistor that can be used in nonvolatile memory devices. One significant hurdle for a suitable resistor is the high temperature activation needs of the diode. The resistor must be able to withstand all subsequent memory device processing without significant loss of resistivity or compromising the remaining materials in the memory device, including any high temperature processing of the memory device.
Moreover, as nonvolatile memory device sizes shrink, it is important to have a resistor that can be easily scaled while still providing the same resistance requirements for the memory device. There is a need to provide a resistor that provides current and future resistance requirements, improves device longevity, and suppresses current passing through the memory device so that the memory device can be properly programmed. Therefore, it is desirable to form a nonvolatile memory device that has low programming currents when switching the device between the “on” and “off” states.